Degradable implantable battery

ABSTRACT

A biodegradable battery is provided. The battery includes an anode comprising a material including an inner surface and an outer surface, wherein electrochemical oxidation of the anode material results in the formation of a reaction product that is substantially non-toxic and a cathode comprising a material including an inner surface and an outer surface, the inner surface of the cathode being in direct physical contact with the inner surface of the anode, wherein electrochemical reduction of the cathode material results in the formation of a reaction product that is substantially non-toxic, and wherein the cathode material presents a larger standard reduction potential than the anode material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part and claims the benefit andpriority of U.S. patent application Ser. No. 13/183,708, filed on Jul.15, 2011, the disclosure of which is hereby incorporated by reference inits entirety.

BACKGROUND

The present disclosure relates to biodegradable implantable batteriesand methods for preparing the same.

Various types of medical devices are designed to be implanted within thehuman body. Examples include stents, scaffolds, drug delivery devices,cardiac rhythm management devices, neurological stimulation devices, andthe like. As electronics, sensors, pacemakers, and cameras areminiaturized it is becoming feasible to deploy smaller devices into thebody.

Some implantable medical devices are designed to function by utilizingenergy from a power source. Such devices often require an on-board powersource, which is implanted into the body as part of the medical deviceand is housed within a sealed case. In many circumstances, the sealedcase is made from a noble metal, such as titanium, to prevent exposureof the contents of the battery to the in vivo environment. This servesto protect the battery from degradation and the patient from toxicbyproducts if the battery leaks, or if the battery is left in the bodyeither inadvertently or during long-term applications.

In the absence of recharging, batteries can only provide a finite amountof power before they are discharged to the point of being useless. Incircumstances where a battery has been completely discharged, thebattery and the associated medical device must generally be explantedand replaced by a new one, unless the battery was only intended fortemporary use. For example, when the battery inside a pacemaker nolonger provides sufficient power, the pacemaker must generally besurgically removed and replaced with a new pacemaker.

Unfortunately, removal of implanted devices is not always an easy task.The body's immunological response to a foreign body generally results inthe formation of fibrous tissue around an implanted medical device overtime. This fibrous tissue can make it difficult to remove implantedmedical devices without risking harm to the patient. Accordingly,implantable batteries that overcome the shortcomings of prior art powersources remain desirable.

SUMMARY

The present disclosure provides a biodegradable battery including ananode comprising a material including an inner surface and an outersurface, wherein electrochemical oxidation of the anode material resultsin the formation of a reaction product that is substantially non-toxicand a cathode comprising a material including an inner surface and anouter surface, the inner surface of the cathode being separated from theinner surface of the anode by a permeable membrane in direct fluidcontact with the aqueous environment in the body; whereinelectrochemical reduction of the cathode material results in theformation of a reaction product that is substantially non-toxic, andwherein the cathode material presents a larger standard reductionpotential than the anode material.

The present disclosure provides for a biodegradable battery according toanother embodiment of the present disclosure. The battery includes ananode comprising a material including an inner surface and an outersurface, wherein electrochemical oxidation of the anode material resultsin the formation of a reaction product that is substantially non-toxic;a cathode comprising a cathode including an inner surface and an outersurface, the inner surface of the cathode being separated from the innersurface of the anode by a permeable membrane in direct fluid contactwith the aqueous environment in the body; and a biodegradable coatingdisposed over the outer surface of the cathode and a portion of theouter surface of the anode, wherein electrochemical reduction of thecathode material results in the formation of a reaction product that issubstantially non-toxic, and wherein the cathode material having alarger standard reduction potential than the anode material.

The present disclosure provides for a biodegradable battery according toa further embodiment of the present disclosure. The battery includes ananode comprising a material including an inner surface and an outersurface, wherein electrochemical oxidation of the anode material resultsin the formation of a reaction products that is substantially non-toxic;a cathode comprising a material, including an inner surface and an outersurface, the inner surface of the cathode being separated from the innersurface of the anode by a permeable membrane in direct fluid contactwith the aqueous environment in the body; a biodegradable coatingdisposed over the outer surface of the cathode and only a portion of theouter surface of the anode; and a hydrogel layer disposed over the outersurface of the anode, the hydrogel layer including at least oneprecursor material, wherein electrochemical reduction of the cathodematerial results in the formation of a reaction product that issubstantially non-toxic, and wherein the cathode material having alarger standard reduction potential than the anode material.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentdisclosure will become more apparent in light of the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1A is a perspective view of a battery according to the presentdisclosure;

FIG. 1B is a cross-sectional view of the battery of FIG. 1A taken alongthe line 1B-1B in FIG. 1A;

FIG. 2A is a perspective view of another battery according to thepresent disclosure;

FIG. 2B is a cross-sectional view of the battery of FIG. 2A taken alongthe line 2B-2B in FIG. 2A;

FIG. 3A is a perspective view of another embodiment of a batteryaccording to the present disclosure;

FIG. 3B is a cross-sectional view of the battery of FIG. 3A taken alongthe line 3B-3B in FIG. 3A;

FIG. 4A is a perspective view of another embodiment of a batteryaccording to the present disclosure; and

FIG. 4B is a cross-sectional view of the battery of FIG. 4A taken alongthe line 4B-4B in FIG. 4A.

DETAILED DESCRIPTION

The present disclosure provides an implantable battery including ananode, a cathode and an optional biodegradable coating. As used herein,the term “biodegradable” in reference to a material shall refer to theproperty of the material being able to be harmlessly absorbed by thebody. In the present application, the terms “biodegradable,”“bioresorbable,” and “bioabsorbable” are used interchangeably and areintended to mean the characteristic according to which a materialdecomposes, or loses structural integrity under body conditions (e.g.,enzymatic degradation or hydrolysis) or are broken down (physically orchemically) under physiologic conditions in the body such that thedegradation products are excretable or absorbable by the body after agiven period of time. The time period may vary, from about one hour toabout several months or more, depending on the chemical nature of thematerial. In embodiments, the material may not be completely absorbed,provided the non-absorbed material poses no health risks and isbiocompatible.

The anode and the cathode utilized in forming the implatantable batteryof the present disclosure include two or more materials havingdissimilar electrochemical potentials, with the anode and the cathodebeing separated by a permeable membrane in direct fluid contact with theaqueous environment in the body. The anode includes an oxidizablematerial that generates electrons and the cathode includes a reduciblematerial that accepts electrons, the cathode material having a largerstandard reduction potential than the anode material. Electrochemicaloxidation of the anode material and reduction then final degradation ofthe cathode material results in the formation of reaction products thatare substantially non-toxic. As used herein the term “substantiallynon-toxic” in reference to a chemical compound shall refer to theproperty of the chemical compound being unlikely to cause harm to anindividual at dosages that are reasonably foreseeable given the mannerin which the chemical compound is being used and/or produced.

Due to the mismatch in potentials, an electrochemical cell between theanode and the cathode is established in which the material possessing amore negative electrochemical potential (e.g., the anode) degrades whilethe other material (e.g., the cathode) remains intact due to cathodicprotection. As current passes from the anode to the cathode, the anodematerial degrades due to galvanic corrosion. The corrosion produces theflow of electrons from the anode to the cathode as the anode reacts withwater and other substances present at the implantation site (e.g., invivo environment). The positive net flow of electrons at the cathodeelevates its potential, thereby protecting the cathode from degradationin an otherwise corrosive environment. Thus, as the anode and thecathode form an electrochemical cell, the anode continuously degradesproviding for the electron flow, while the cathode is cathodicallyprotected, thereby generating current.

The battery according to the present disclosure may generate a voltagepotential from about 0.01 volts (V) to about 2.5 V, in embodiments fromabout 0.01 V to about 1.0 V, and may have a capacity of from about 0.058milliampers per hour per cm² (mAh/cm²) to about 66.5 mAh/cm², inembodiments from about 1 mAh to about 40 mAh. Once the anode hascompletely degraded, the current generation ceases and the cathode is nolonger cathodically protected, resulting in subsequent degradation ofthe cathode material and the battery.

The amount of the anode material may be from about 40% to about 99%,from about 90% to about 99%, in further embodiments from about 95% toabout 99% by weight of the combined mass of the anode and the cathode.The amount of the cathode material may be from about 1% to about 60%, inembodiments from about 1% to about 10%, in further embodiments fromabout 1% to about 5% by weight of the combined mass of the anode and thecathode.

The anode can be made of various metals, various alloys of metals,compounds including metal atoms, ceramic/metal composites, variouspolymers, and combinations thereof. Suitable metals and alloys forforming the anode include, but are not limited to, calcium, magnesium,iron, bismuth, zinc, electrochemically oxidizable degradable polymerssuch as organometallic polymers, their alloys, and combinations of anyof the foregoing. Additional first materials may include polypyrrolebased positive electrode arrays, carbon microelectromechanical systems,iron oxide carbon nanofibers, lithium based anodes such as lithiumtitanium phosphate, metal oxides such as zinc oxide with conductiveceramics such as magnesium hydroxide and calcium hydroxide,nanocomposites, cobalt iron oxides, combinations thereof, and the like.

The cathode may include materials including, but not limited to, metaloxides, metal hydroxides, metal oxyhydroxides, polyoxymetallates, metalsalts, electrochemically reducible organic compounds, electrochemicallyreducible bioresorbable polymers, and combinations thereof. Suitablemetal oxides include, but are not limited to, manganese oxides (e.g.,Mn₂O₃ and MnO₂) iron(III) oxide, bismuth oxides (e.g., Bi₂O₃ and Bi₂O₄)and combinations thereof. Suitable metal oxyhydroxides include, but arenot limited to, manganese oxyhydroxide. Suitable metal salts include,but are not limited to, metal halides such as iron chloride, metalsulfides such as iron sulfide and bismuth sulfide, metal sulfates suchas iron sulfate and manganese sulfate, metal phosphates such as ironphosphate and manganese phosphate, and combinations thereof.

Suitable electrochemically reducible organic compounds for use as acathode include, but are not limited to, electrochemically activebiological compounds, including metalloenzymes and metalloproteins, suchas ferredoxins, oxidases (e.g., cytochrome c oxidase), peroxidases,catalases, superoxide dismutases, metal ion containing macrocyclecompounds, such as porphyrins, phthalocyanines, tetraazamacrocycles, andcombinations thereof.

Suitable electrochemically reducible bioresorbable polymers include, butare not limited to, non-toxic conjugated or nonconjugated polymericdisulfide compounds, conjugated or nonconjugated metallopolymers basedon nontoxic metal ion complexes, such as ferrocene (includingpolyvinylferrocene), Schiff bases or heterocycle metal ion complexes,such as porphyrins, phthalocyanines, tetraazamacrocycles, andcombinations thereof. Suitable electrochemically reducible bioresorbablepolymers also include conjugated organic polymers such as polypyrroles,polythiophenes, polyanilines, polyethylene oxide, polylactide,polycaprolactone, and combinations thereof. Other cathode materialsinclude, but are not limited to, sodium iron oxide, potassium ironmanganese oxide, calcium zincate, zinc manganese oxide, Birnessite typemanganese oxide, iron-manganese phosphate composites, sodium orpotassium cobalt manganese calcium oxide, olivine, spherical spinels,spinels, polypyrrole, maghemite, and combinations thereof. Inembodiments, the cathode may be a dual active material composite cathodeformed from sulfur and oxide.

Exemplary pairings of anode and cathode materials include, but are notlimited to, those described in Table 1 below.

TABLE 1 Anode material Cathode material Mg MnOOH Mg FePO₄ Mg FerredoxinMg Polyethylenedisulfide Fe FeOOH

The battery may also include a biodegradable coating disposed about thebattery, which may be formed from any suitable biodegradable polymer.The coating covers a majority of the exposed surface of the cathode andmay include one or more apertures therein to provide for a fluidexchange with the anode and permeable membrane between the cathode andanode. In embodiments, the coating may also expose the cathode to thebody fluids to maintain charge balance and provide for current flow.Thus, the coating exposes only the anode to the corrosive environmentwhile encasing the cathode. The coating may be degradable at apredetermined rate and/or only at desired surfaces. In embodiments, thecoating may be of varying thicknesses across different surfaces of thebattery.

Suitable biodegradable polymers which may be used to form a coating of abattery of the present disclosure include polymers such as aliphaticpolyesters; polyamides; polyamines; polyalkylene oxalates;poly(anhydrides); polyamidoesters; copoly(ether-esters);poly(carbonates) including tyrosine derived carbonates;poly(hydroxyalkanoates) such as poly(hydroxybutyric acid);poly(hydroxyvaleric acid); and poly(hydroxybutyrate); polyimidecarbonates; poly(imino carbonates) such as poly (bisphenolA-iminocarbonate and the like); polyorthoesters; polyoxaesters includingthose containing amine groups; polyphosphazenes; poly (propylenefumarates); polyurethanes; polymer drugs such as polydiflunisol;polyaspirin; and protein therapeutics; biologically modified (e.g.,protein; peptide) bioabsorbable polymers; and combinations thereof.

More specifically, for the purpose of this disclosure, aliphaticpolyesters which may be utilized include, but are not limited to,homopolymers and copolymers of lactide (including lactic acid, D-L- andmeso lactide), glycolide (including glycolic acid),epsilon-caprolactone, p-dioxanone(1,4-dioxan-2-one), trimethylenecarbonate(1,3-dioxan-2-one), alkyl derivatives of trimethylenecarbonate, Δ-valerolactone, β-butyrolactone, γ-butyrolactone,ε-decalactone, hydroxybutyrate, hydroxyvalerate, 1,4-dioxepan-2-one(including its dimer 1,5,8,12-tetraoxacyclotetradecane-7,14-dione),1,5-dioxepan-2-one, 6,6-dimethyl-1,4-dioxan-2-one, 2,5-diketomorpholine,pivalolactone, α,α diethylpropiolactone, ethylene carbonate, ethyleneoxalate, 3-methyl-1,4-dioxane-2,5-dione,3,3-diethyl-1,4-dioxan-2,5-dione, 6,8-dioxabicycloctane-7-one, andcombinations thereof.

Other suitable biodegradable polymers include, but are not limited to,poly(amino acids) including proteins such as collagen (I, II and III),elastin, fibrin, fibrinogen, silk, and albumin, peptides includingsequences for laminin and fibronectin (especially its RGD site),polysaccharides such as hyaluronic acid (HA), dextran, alginate, chitin,chitosan, and cellulose, glycosaminoglycan, gut, and combinationsthereof. Collagen as used herein includes natural collagen such asanimal derived collagen, gelatinized collagen, or synthetic collagensuch as human or bacterial recombinant collagen.

Additionally, synthetically modified natural polymers such as celluloseand polysaccharide derivatives, including alkyl celluloses, hydroxyalkylcelluloses, cellulose ethers, cellulose esters, nitrocelluloses, andchitosan may be utilized as coating of a battery of the presentdisclosure. Examples of suitable cellulose derivatives include methylcellulose, ethyl cellulose, hydroxypropyl cellulose, hydroxypropylmethyl cellulose, hydroxybutyl methyl cellulose, cellulose acetate,cellulose propionate, cellulose acetate butyrate, cellulose acetatephthalate, carboxymethyl cellulose (CMC), cellulose triacetate,cellulose sulfate sodium salt, and combinations thereof. These may becollectively referred to herein, in embodiments, as “celluloses.”

In embodiments, the coating may be a hydrogel disposed on the surface ofthe anode, thereby providing for a controlled exposure of the anode tothe fluids present in vivo. This slows the degradation rate of the anodeand prolongs the longevity of the battery. Suitable hydrogels are withinthe purview of those skilled in the art and include, for example, thosediscussed in U.S. Pat. Nos. 6,184,266, 6,368,356, and 7,605,232, theentire disclosures of each of which are incorporated by referenceherein.

The coating may be present in an amount from about 500 nm to about 2 mm,in embodiments from about 1 μm to about 500 μm in thickness. Thicker andthinner coatings may also be possible. Denser, more water impermeablecoatings may be thinner, while porous, more water permeable coatings maybe thicker to cause slowing of degradation rate. Battery longevity maybe controlled by adjusting the degradation rate and the mass of theanode. Current delivered by the battery depends on the degradation rateof the anode which, in turn, depends on the surface area exposed to thecorrosive in vivo environment. In embodiments, the surface area of thebattery may also be adjusted by modifying the shape of the anode and thecathode, and/or providing a coating, to tailor the exposed surface toachieve a desired degradation rate.

Referring now to FIGS. 1A and 1B, perspective and cross-sectional viewsof a battery assembly 100, respectively. The battery assembly 100includes an anode 102, and a cathode 106. A permeable membrane 101 isdisposed between the anode and cathode. The anode 102 has asubstantially rectangular cross-sectional shape and the cathode 106 isdisposed as a relatively thin layer on the surface of the anode 102.

During operation, an electrochemical oxidation reaction takes place atthe anode 102 liberating electrons that can then be used to drive a load(e.g., a medical device). The anode material can be selected so that thereaction products of the electrochemical oxidation are substantiallynon-toxic. In this manner, the reaction products at the anode can simplybe absorbed by the body.

The cathode 106 may also be formed from a material that whenelectrochemically reduced forms products that are substantiallynon-toxic. Cathode materials disclosed herein may be selected so thatthe cathode material has a higher oxidation potential than the materialof the anode. In other words, the cathode material may be selected sothat is has a larger standard reduction potential than the material ofthe anode.

In embodiments, the cathode material may be a material configured tointercalate or otherwise capture ions produced during electrochemicaloxidation of the anode, such that if electrochemical oxidation of theanode material results in the formation of metal (e.g., magnesium) ions,the cathode material can be selected to be a material that canintercalate metal ions. In further embodiments, the cathode material maybe a material configured to intercalate ions other than those producesby the anode 102, such as sodium and/or hydrogen ions.

The anode 102 is described as being composed of magnesium and thecathode 106 being composed of manganese (IV) oxide. Without being boundby any particular theory, when the magnesium is electrochemicallyoxidized, magnesium ions are formed and then absorbed into the body.Magnesium is a required nutrient and as such is substantially non-toxicat the concentrations that would likely be achieved by embodiments asdisclosed herein. When the manganese (IV) oxide is electrochemicallyreduced, manganese (III) oxide is formed, which may be further reducedto manganese (II) hydroxide and then absorbed by the body.

As shown in FIGS. 1A and 1B, the anode 102 is in physical and/orelectrochemical contact with the cathode 106. In embodiments, thebattery 100 may include a plurality of cells including a plurality ofanodes 102 and cathodes 106. The anode 102 includes an inner surface103. The anode 102 and the cathode 106 also include outer surfaces 105and 109, respectively. As used herein, the term “inner surface” denotessurfaces of the anode 102 and the cathode 106 that are facing eachother, whereas the term “outer surface” denotes outward surfaces facingthe in vivo environment. The outer surfaces 105 and 109 are exposed tothe in vivo environment aiding in the degradation of the anode 102 andthe cathode 106 and the battery 100. The anode 102 also includes a firstconductor 104 and the cathode 106 includes a second conductor 108. Eachof the conductors 104 and 108 is coupled to a load (not shown).

The current generated between the anode 102 and the cathode 106 may beused to power any number of implantable devices. The load may be anytemporary or permanently implanted medical device that utilizes a powersource and may include both biosorbable and non-biosorbable medicaldevices. Exemplary medical devices may include, but are not limited to,neurological stimulators, implantable sensors, implantable cardiacrhythm management devices, such as pacemakers, cardiac resynchronizationtherapy (CRT) devices, remodeling control therapy (RCT) device,cardioverter/defibrillators, or pacemaker-cardioverter/defibrillators,and the like.

When the battery 100 is activated, a current is generated and isdelivered to the load through the conductors 104 and 108. Specifically,electrons flow from the anode 102 in the direction of arrow 110 throughthe first conductor 104. The electrons can then pass through the load,through the second conductor 108 in the direction of arrow 112, beforecompleting the circuit at the cathode 106. In order to maintain chargebalance, positively charged ions move from the anode 102 to the cathode106.

During the discharging process, the anode 102 dissolves as the anodematerial is electrochemically oxidized. In embodiments where the anodematerial is magnesium, the anode 102 is broken down according to thefollowing reaction represented by formula (I):

Mg→Mg₂ ⁺+2e⁻  (I)

In embodiments where the cathode 106 is manganese (IV) oxide, thecorresponding half-cell reaction at the cathode is described by thefollowing half-cell reaction represented by formula (II):

2MnO₂+H₂O+2e⁻→Mn₂O₃+2OH⁻  (II)

As the anode 102 reacts, a soluble species (e.g., Mg²) is formed at theanode 102 which dissolves into the extracellular fluid of the body. Asdescribed above, the cathode 106 is cathodically protected fromdegradation. After the anode 102 is completely degraded, the cathode 106is no longer cathodically protected and degrades as follows: manganese(III) oxide forms at the cathode 106, which can be further reduced tomanganese (II) hydroxide and then dissolves in the extracellular fluidof the body. As such, the anode 102 and cathode 106 erode and arecompletely dissolved, resulting in degradation of the battery 100.

In embodiments, electrochemical reduction at the cathode 106 forms achemical species that, while not highly soluble in the aqueous in vivoenvironment, nonetheless subsequently breaks down chemically anddissolves after the battery 100 is no longer operational. As such, evenin these circumstances, the components of the battery are absorbed bythe body.

FIGS. 2A and 2B, show perspective and cross-sectional views of analternate battery 200, respectively. The battery 200 includes an anode202 and a cathode 206. The anode 202 is in physical and/orelectrochemical contact with the cathode 206. The anode 202 includes aninner surface 203. The anode 202 and the cathode 206 also include outersurfaces 205 and 209, respectively. The anode 202 also includes a firstconductor 204 and the cathode 206 includes a second conductor 208. Eachof the conductors 204 and 208 is coupled to the load. Current flows fromthe anode 202 in the direction of arrow 210 through the first conductor204. The current then passes through the load, through the secondconductor 208 in the direction of arrow 212, before completing thecircuit at the cathode 206.

The battery 200 also includes a biodegradable coating 214 disposed overthe entire outer surface 209 of the cathode 206 and a portion of theouter surface 205 of the anode 202. The coating 214 includes one or moreopenings 216 exposing a portion of the anode 202 to the in vivoenvironment. This arrangement protects the cathode 206 from degradationand exposes only the anode 202 and in embodiments, the cathode 206, tothe corrosive environment. The coating 214 may be degradable at apredetermined rate. The coating 214 may fully degrade from about 2 hoursto about 2 yrs, in embodiments from about 2 hours to about 1 yr.

In embodiments, the coating 214 does not completely degrade faster thanthe anode 202. Alternatively, the coating 214 could last only until thebattery 200 is activated. In further embodiments, a series of batteries200 may be implanted, each with a coating 214 having a differentdegradation time so as to extend the time over which a device ispowered. In this embodiment, each battery 200 in the series wouldactivate as the proceeding battery is exhausted. For a device requiringvery low power and a slow degrading battery the activation time could befrom a few hours for the first battery 200 to several months or more forthe last battery 200 to be activated.

The coating 214 may degrade at different rates on different portions ofthe outer surfaces 205 and 209. In embodiments, the portion of thecoating 214 covering the outer surface 209 of the cathode 206 maydegrade at a slower rate than the portion of the coating 214 coveringthe outer surface 205 of the anode 202, allowing the anode 202 to bedegraded prior to the cathode 206 being exposed to the in vivoenvironment. This may be accomplished by adjusting the thickness of thecoating 214 and/or the chemical constituents thereof within variousportions thereof.

FIGS. 3A and 3B show perspective and cross-sectional views of yetanother alternate battery 300, respectively. The battery 300 includes ananode 302 and a cathode 306. The anode 302 is separated from the cathode306 by a membrane such as membrane 101. The anode 302 includes an innersurface 303 separated from an inner surface 307 of the cathode 306. Theanode 302 and the cathode 306 also include outer surfaces 305 and 309,respectively. The anode 302 also includes a first conductor 304 and thecathode 306 includes a second conductor 308. Each of the conductors 304and 308 is coupled to the load. Current flows from the anode 302 in thedirection of arrow 210 through the first conductor 304. The current thenpasses through the load, through the second conductor 308 in thedirection of arrow 312, before completing the circuit at the cathode306.

The battery 300 also includes a biodegradable coating 314 disposed overthe entire outer surface 309 of the cathode 306. As shown, the coating314 only covers the cathode 306. In embodiments, the coating 314 mayalso cover a portion of the outer surface 305 of the anode 302. Thecoating 314 is similar to the coating 214, including similar degradationproperties, and it may be composed of similar constituents.

In embodiments, the battery 300 may further include a coating layer 316disposed over the outer surface 305 of the anode 302. The coating layer316 may be any material described above that is water-permeable or maybe a water-permeable hydrogel, providing for a controlled exposure ofthe anode 302 to the fluids present in vivo, thereby controlling thedegradation rate of the anode 302 and prolonging the longevity of thebattery 300.

FIGS. 4A and 4B, show perspective and cross-sectional views of anotherbattery assembly 400, respectively. The battery 400 includes an anode402 and a cathode 406. The anode 402 has a substantially frustoconicalcross-sectional shape with the cathode 406 being disposed as a foillayer on an inner surface 403 of the anode 402. The anode 402 isseparated from the cathode 406 by a membrane such as membrane 101. Theanode 402 includes an inner surface 403. The anode 402 and the cathode406 also include outer surfaces 405 and 409, respectively. The anode 402also includes a first conductor 404 and the cathode 406 includes asecond conductor 408. Each of the conductors 404 and 408 is coupled tothe load. Current flows from the anode 402 in the direction of arrow 410through the first conductor 404. The current then passes through theload, through the second conductor 408 in the direction of arrow 412,before completing the circuit at the cathode 406.

The battery 400 also includes a biodegradable coating 414 disposed overthe entire outer surface 409 of the cathode 406. As shown the coating414 only covers the cathode 406. In embodiments, the coating 414 mayalso cover a portion of the outer surface 405 of the anode 402. Thecoating 414 is substantially similar to the coating 214 includingsimilar degradation properties and may be composed of similarconstituents. The frustoconical shape of the battery 400 minimizes thesurface area of the outer surface 405 of the anode 402. The dimensions(e.g., angle) of the sides of the battery 400 may be tailored to obtaina desired exposure of the anode 402 to the in vivo environment andthereby control the degradation rate thereof. The conical orfrustoconical shape provides for a greater surface area of the anode 402to be exposed as it degrades, thus impacting current supply as afunction of implant time.

It will be appreciated that, in some embodiments, the tissue of the bodyitself can be utilized for the purpose of transferring ions between theanode and the cathode. For example, the extracellular fluid in vivo isan aqueous solution that includes at least some amount of electrolytessuch as sodium and potassium. Therefore, extracellular fluid can be usedto transfer ions between the anode and the cathode in embodiments. It isthe expectation that in vivo fluids will transfer ions between thecathode and anode, but other means can be used.

It will be appreciated that the above-disclosed and other features andfunctions, or alternatives thereof, may be desirably combined into manyother different systems or applications. Also that various presentlyunforeseen or unanticipated alternatives, modifications, variations orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.Unless specifically recited in a claim, steps or components of claimsshould not be implied or imported from the specification or any otherclaims as to any particular order, number, position, size, shape, angle,or material.

What is claimed is:
 1. A biodegradable battery comprising: an anodecomprising a material including an inner surface and an outer surface,wherein electrochemical oxidation of the anode material results in theformation of a reaction product that is substantially non-toxic; and acathode comprising a material including an inner surface and an outersurface, the inner surface of the cathode being separate from the innersurface of the anode, wherein electrochemical reduction of the cathodematerial results in the formation of a reaction product that issubstantially non-toxic, and wherein the cathode material presents alarger standard reduction potential than the anode material.
 2. Thebiodegradable battery of claim 1, wherein the anode material is selectedfrom the group consisting of magnesium, iron, zinc, electrochemicallyoxidizable degradable polymers, alloys, and combinations thereof.
 3. Thebiodegradable battery of claim 1, wherein the cathode material isselected from the group of materials consisting of metal oxides, metalhydroxides, metal oxyhydroxides, polyoxymetallates, metal salts,electrochemically reducible degradable polymers, and combinationsthereof.
 4. The biodegradable battery of claim 1, further comprising abiodegradable coating disposed over the outer surface of the cathode anda portion of the outer surface of the anode.
 5. The biodegradablebattery of claim 4, wherein the biodegradable coating comprises apolymer selected from the group consisting of aliphatic polyesters,polyamides, polyamines, polyalkylene oxalates, poly(anhydrides),polyamidoesters, copoly(ether-esters), poly(carbonates),poly(hydroxyalkanoates), poly(hydroxyvaleric acid),poly(hydroxybutyrate), polyimide carbonates, poly(imino carbonates),polyorthoesters, polyoxaesters, polyphosphazenes, poly(propylenefumarates), polyurethanes, polymer drugs, and combinations thereof. 6.The biodegradable battery of claim 1, further comprising a hydrogellayer disposed over the outer surface of the anode, the hydrogel layercomprising at least one precursor material.
 7. A biodegradable batterycomprising: an anode comprising a material including an inner surfaceand an outer surface, wherein electrochemical oxidation of the anodematerial results in the formation of a reaction product that issubstantially non-toxic; a cathode comprising a cathode including aninner surface and an outer surface, the inner surface of the cathodebeing separate from; and a biodegradable coating disposed over the outersurface of the cathode and a portion of the outer surface of the anode,wherein electrochemical reduction of the cathode material results in theformation of a reaction product that is substantially non-toxic, andwherein the cathode material having a larger standard reductionpotential than the anode material.
 8. The biodegradable battery of claim7, wherein the anode material comprises a material selected from thegroup consisting of magnesium, iron, zinc, electrochemically oxidizabledegradable polymers, alloys and combinations thereof.
 9. Thebiodegradable battery of claim 7, wherein the cathode material comprisesa material selected from the group of materials consisting of metaloxides, metal hydroxides, metal oxyhydroxides, polyoxymetallates, metalsalts, electrochemically reducible degradable polymers, and combinationsthereof.
 10. The biodegradable battery of claim 7, wherein thebiodegradable coating comprises a polymer selected from the groupconsisting of aliphatic polyesters, polyamides, polyamines, polyalkyleneoxalates, poly(anhydrides), polyamidoesters, copoly(ether-esters),poly(carbonates), poly(hydroxyalkanoates), poly(hydroxyvaleric acid),poly(hydroxybutyrate), polyimide carbonates, poly(imino carbonates),polyorthoesters, polyoxaesters, polyphosphazenes, poly(propylenefumarates), polyurethanes, polymer drugs, and combinations thereof. 11.The biodegradable battery of claim 7, further comprising a hydrogellayer disposed over the outer surface of the anode, the hydrogel layercomprising at least one precursor material.
 12. The biodegradablebattery of claim 7, wherein the biodegradable battery has asubstantially frustoconical shape.
 13. A biodegradable batterycomprising: an anode comprising a material including an inner surfaceand an outer surface, wherein electrochemical oxidation of the anodematerial results in the formation of a reaction products that issubstantially non-toxic; a cathode comprising a material, including aninner surface and an outer surface, the inner surface of the cathodebeing separate from the inner surface of the anode; a biodegradablecoating disposed over the outer surface of the cathode; and a hydrogellayer disposed over the outer surface of the anode, whereinelectrochemical reduction of the cathode material results in theformation of a reaction product that is substantially non-toxic, andwherein the cathode material having a larger standard reductionpotential than the anode material.
 14. The biodegradable battery ofclaim 13, wherein the anode material comprises a material selected fromthe group consisting of magnesium, iron, zinc, electrochemicallyoxidizable degradable polymers, alloys and combinations thereof.
 15. Thebiodegradable battery of claim 13, wherein the cathode materialcomprises a material selected from the group of materials consisting ofmetal oxides, metal hydroxides, metal oxyhydroxides, polyoxymetallates,metal salts, electrochemically reducible degradable polymers, andcombinations thereof.
 16. The biodegradable battery of claim 13, whereinthe biodegradable coating comprises a polymer selected from the groupconsisting of aliphatic polyesters, polyamides, polyamines, polyalkyleneoxalates, poly(anhydrides), polyamidoesters, copoly(ether-esters),poly(carbonates), poly(hydroxyalkanoates), poly(hydroxyvaleric acid),poly(hydroxybutyrate), polyimide carbonates, poly(imino carbonates),polyorthoesters, polyoxaesters, polyphosphazenes, poly(propylenefumarates), polyurethanes, polymer drugs, and combinations thereof. 17.The biodegradable battery of claim 13, wherein the biodegradable coatingis disposed over only a portion of the outer surface of the anode. 18.The biodegradable battery of claim 13, further comprising a permeablemembrane in direct fluid contact with the aqueous environment of apatient's body.